MENTAL OPERATIONS IN VISION. 589 



said to be red-blind. Moreover, if we investigate our negative 

 after images, after looking for a long time at a red object, we 

 find them to be greenish-blue ; that is to say, the nervous mech- 

 anism for receiving red impressions is fatigued, and consequently 

 those of its complementary color are easily stimulated. 



MENTAL OPERATIONS IN VISION. 



Our visual sensations enable us to perceive the existence, the 

 position, and the form of the various objects around us. For the 

 perfection of a visual perception much more is necessary than 

 the mere perfection of the dioptric media of the eye, and of the 

 retinal nerve-mechanisms. Besides the changes produced in the 

 retina by the light, by means of which the optic nerve is stimu- 

 lated, and the excitations produced, by the impulses passing along 

 the nerve, in the nerve cells of the seeing centre, there must be 

 further a psychical action in other cells of the cortex of the brain. 

 This psychical action of the brain consists of a series of conclu- 

 sions drawn from the experiences gained by our visual and other 

 sensations. 



Our ideas of external objects are not in exact accord with the 

 image produced on the retina and transmitted to the brain, but 

 are the result of a kind of argument carried on unconsciously in 

 our minds. Thus when no light reaches the retina, we say (with- 

 out what we call thought) that it is dark ; our retina being un- 

 stimulated, no impulse is communicated, and the sensation of 

 blackness arises in our sensorium. When luminous rays are re- 

 flected to the retina from various objects around us, the physio- 

 logical impulse starts from the eye, but in the brain, by uncon- 

 scious psychical activity, it is referred in our minds to the objects 

 around us, so that mentally we project into the outer world what 

 really occurs in the eye. So also, from habit, we reinvert in our 

 minds the image which is thrown on the retina by the lens, upside- 

 down, and so unconscious are we of the psychical act that we 

 find it hard to believe that our eyes really see everything in- 

 verted, and our minds have to reinstate them in the upright 

 position. 



